This invention relates to a crank device for use in the crank-pedal mechanism of a bicycle or for use as a coupling member between a piston and the crankshaft of an automotive engine.
FIG. 4B shows a conventional crank device of this type. It has a crank arm 111 having one end thereof coupled to a rotary shaft 112. Driving force F is applied to the other end of the arm 111.
The arm 111 of this conventional crank device has a fixed length L', so that it makes a completely circular movement about the rotary shaft 112. Torque T is thus produced at the rotary shaft 112.
Let us now consider the crank-pedal mechanism of a bicycle. Force is applied to the pedal only while the pedal is moving down, i.e. only in a range about half the full one rotation cycle. In the other half of the cycle, the crank arm is rotated by inertia with no external force being applied.
Similarly, in case of an automotive engine, driving force acts on the crank arm not throughout the entire range of the rotation cycle of the crank arm but only while the piston is being accelerated by the combustion of the air-fuel mixture in the cylinder. At other times, the crank arm is rotated by inertia.
In order to produce as large torque T as possible with the conventional crank device shown in FIG. 4B, the length L' of the crank arm 111 has to be correspondingly large. But a long crank arm leads to an increase in the size of the entire crank device as well as its working space.
If the crank-pedal mechanism of a bicycle has such long crank arms 111, the rider will have a hard time exerting a force on the pedals because the turning radius of the pedals is very long and thus he has to move his feet along a correspondingly large circular path.